A resonant cavity is a device having an enclosed volume bounded by electrically conductive surfaces and in which oscillating electromagnetic fields are sustainable. Resonant cavities may be used filters, for example, and have excellent power handling capability and low energy losses. Several resonant cavities may be coupled together to achieve sophisticated frequency selective behavior.
Resonant cavities are often milled in, or cast from, metal. The frequency of operation determines the size of the cavity required, and, in the microwave range, the size and weight are significant. In a re-entrant resonant cavity, the electric and magnetic parts of the electromagnetic field within the cavity volume are essentially geometrically separated, enabling the size of the cavity to be reduced compared to that of a cylindrical cavity having the same resonance frequency.
Since the geometrical shape of a resonant cavity determines its frequency of resonance, high mechanical accuracy is required and, in addition, or alternatively, post-production tuning is applied. For example, tuning mechanisms may be provided, such as tuning screws that project into the cavity volume by a variable amount and are adjusted manually. FIG. 1 schematically illustrates a re-entrant resonant cavity 1 which includes a manually adjusted tuning mechanism. The cavity 1 has an enclosed volume 2 defined by a cylindrical outer wall 3, end walls 4 and 5, and a re-entrant stub 6 extensive from one of the end walls 4. The electric field concentrates in the capacitive gap 7 between the end face 8 of the stub 6 and part 9 of the cavity wall 5 facing it. The end face 8 includes a blind hole 10 aligned with the longitudinal axis X-X of the stub 6. A tuning screw 11 projects from the end wall 5 into the hole 10. Energy is coupled into the resonant cavity and an operative monitors the effect on resonant frequency as he moves the tuning screw 11 in an axial direction relative to the end face 8, as shown by the arrow, to alter the value of the capacitance of the capacitive gap. This enables the resonance frequency of the cavity to be adjusted to the required value.
One known method for reducing the weight of a cavity is to manufacture it in plastic and cover its surface with a thin metal film. If milling is used to shape the plastic, it can be difficult to achieve sufficient accuracy, and surface roughness may be an issue. Molding is another approach, but the tooling is expensive, particularly when the cavities are combined together as a filter. In a typical multi-resonator filter, for example, the resonance frequencies of most of the included resonators differ from one another. The filter functionality requires slightly different resonance frequencies and therefore slightly different geometries for the resonators. As a consequence, if molding techniques are used, for example, plastics injection molding, a single molding form must be configured to define all of the resonators. Such a complex form is difficult to produce with sufficient accuracy, and hence incurs significant costs.
T. J. Mueller, “SMD-type 42 GHz waveguide filter”, Proc. IEEE Intern. Microwave Symp., Philadelphia, 2003, pp. 1089-1092 describes manufacture of a waveguide filter using surface mount soldering in which a U-shaped metal filter part is soldered onto a printed circuit board (PCB), using the board metallization to define one of the waveguide walls.